This invention relates to an apparatus for encoding television signals, and more specifically to television signal encoding devices of the type utilizing correlation between frames for data compression or reduction in redundancy of information to be transmitted.
Known as an encoding technique which enables transmission of television signals using, on the average, only one bit or so per sampled element is an interframe coding method such as the one proposed by J. C. Candy et al. in the article entitled "Transmitting Television as Clusters of Frame-to-Frame Differences, " The Bell System Technical Journal, Vol. 50, No. 6 (July - August, 1971), pp 1889 - 1917. The basic concept of the so-called interframe coding is to encode and transmit only information on "significant" picture-elements, i.e., those elements whose intensity changes between successive frames of television signal in amounts exceeding a certain threshold value. With this method, a substantially high ratio of compression and redundancy reduction can be obtained by setting the threshold at a rather high value in order to materially reduce the number of significant picture-elements. Obviously, the number of significant changes occurring between successive frames is limited in scenes with little activity but increases as motion in the scene increases. On the other hand, the number of significant elements decreases as the threshold value is raised. As for the picture quality, however, raising the threshold value increases the coding error, causing impairments in the picture. This makes it desirable to set the threshold at a lower value. Under this situation, the threshold value is usually changed in interframe coding to control the amount of coded information in accordance with the rate of occurrence of significant picture-elements.
In order to realize any high quality television transmission system by such interframe coding technique, it is necessary to lower the threshold value and raise the transmission bit rate. As the threshold value is reduced, however, the number of unwanted significant picture-elements occurring on account of the noise and small amplitude variations included in the television signal, increases and the amount of information to be transmitted starts to increase suddenly at a definite threshold value. Because of this, it has previously been practically infeasible to reduce the threshold value below a certain level and there has been a definite limitation to reduction of the coding error associated with determination of significant picture-elements.